Methods of manufacturing vertical cavity surface emitting laser (VCSEL) chips are generally known. However, one of the impediments to the successful manufacturing of such devices is the susceptibility of such devices to damage. In addition to physical damage, VCSELs and photodiodes may be subject to electrostat discharge (ESD) or electrical over-stress (EOD) damage.
Two market trends are driving VCSEL and photodiode chip design towards a regime where die damage during handling is more and more likely. The first trend has to do with smaller active regions. Smaller active regions (volumes) are needed for higher speeds (e.g., 4 Gb/s, 10 Gb/s applications), modal control (e.g., sensing applications, optical mice, etc.) or both. ESD/EOS damage thresholds fall as one over the square of the optical aperture size of such devices.
In addition to smaller active regions, a second trend has to do with a smaller overall die (chip) size. Smaller chips are needed to reduce die manufacturing costs to meet market demanded prices.
To assemble separated die into packages, the die must be picked off of the wafer then placed into the final package. As chip size shrink, less chip area is allowed for the pickup tool. This means that die are being touched closer to their active regions where mechanical damage or ESD strikes are more likely to compromise chip reliability. The rate at which die fail early due to manufacturing damage is typically referred to as the infant mortality rate.
Statistics have shown that once installed, the average expected lifetime of conventional VCSELs are significantly longer than the devices in which the VCSELs are to be used. However, during manufacturing and before installation, VCSELs and photodiodes suffer a relatively high infant mortality rate due to handling practices. Mechanical damage and ESD strikes during chip handling, in fact, have been demonstrated to be the leading cause of infant mortality.
In addition to the direct loss due to infant mortality, additional efforts must be expended to identify damaged devices. For example, visual inspections are often performed to screen out die with mechanical damage.
However, visual inspection is imprecise, expensive and time consuming. Typically, the visual defects that must be screened out are either very small or very subtle.
Various electrical tests may be performed to identify die that have suffered ESD/EOS damage. In some cases, packaged die may be burned-in at currents and ambient temperatures above what would be expected during use in an attempt to identify damaged die.
Electrical tests for ESD damage are effective if properly performed and interpreted. However, they are also time consuming and expensive.
In addition, visual and/or electrical testing may not always be practical. For example, future applications, such as optical computer mice, are expected to be extremely price sensitive. Burn-in of all or some portion of packaged parts may not be economically feasible. Accordingly, a need exists for methods of reducing the root causes for infant mortality due to ESD and or reducing the need for visual inspections.